Chemical/thermal modification of poly(vinyl alcohol) film for enhanced water vapour barrier properties.

Within the packaging industry, the increasing demand for sustainable packaging is driving research towards renewable coating materials for paper or paperboard with high barrier properties against gas, water vapour and odours. Poly(vinyl alcohol) (PVOH), a water soluble and biodegradable polymer, is a real option for sustainable packaging when applied as either a coating for paper and paperboard packaging or an independent packaging film. Since its application is limited in high humid environments, several modification methods including chemical crosslinking with glutaraldehyde (GA), salt treatment with sodium sulphate solution, heat treatment and nanoclay incorporation have been investigated in order to improve its barrier to water vapour and also thermal stability and mechanical properties. An extensive range of crosslinking times between GA and PVOH have been assessed. The crosslinked PVOH and salt treated films show an improvement in water vapour barrier properties by 60-70%. Whereas, heat treatment and clay addition show an improvement of 20-57%, in which the water vapour barrier properties increase with increasing heat treatment temperatures (40°C to 180°C) or clay contents (5 to 20 wt%). Additionally, crosslinking PVOH/Clay films with GA improves their water vapour barrier properties comparable to those of crosslinked PVOH films. Apart from the heat treated films, all the modified PVOH films possess higher thermal stability than neat PVOH films as evidenced by thermogravimetric analysis measurements. Combinations between heat treatment and crosslinking with GA as well as crosslinking on one side of PVOH films have been investigated. The films annealed at 180°C prior to crosslinking on both sides do not dissolve in hot water (90°C) even with short crosslinking time (5 minutes) and their water vapour barrier properties derive mainly from their enhanced crystallinity. The respective one-side crosslinked PVOH films show comparable water vapour barrier properties and thermal stability, but were dissolvable in hot water.The diffusion of crosslinking solution into the PVOH films has been studied using in-situ FTIR. It has been shown that the crosslinking solution can rapidly penetrate and diffuse from the top to the bottom of the film (50-60 ?m).The crystallinity of PVOH films after subjection to different modification methods have been investigated using various techniques, including FTIR, Raman and XRD. It has been shown that the crystallinity increases with heat treatment whilst decreases after crosslinking with GA. On the contrary, treating PVOH films with sodium sulphate solution for different lengths of time did not change the crystallinity of the films. When clay is present in PVOH films with 5 wt%, the crystallinity is not affected but increases slightly and significantly with 10 and 20 wt% clay loading, respectively.